In general, a rotary compressor is used for an air conditioner. As functions of the air conditioner are diversified, a rotary compressor that can vary its capacity is being required.
As techniques for varying the capacity of the rotary compressor, what so called an inverter method of controlling the revolutions of the compressor by employing an inverter motor has been well known. However, this technique is problematic for the following reasons. First, the inverter motor itself is expensive, which causes an increase in unit cost. Also, even though most air conditioners are used as cooling devices, improving cooling capability under the cool circumstance is more difficult than improving the capability under the warm circumstance.
For this reason, instead of the inverter method, “a technique of varying the capability of compressing a refrigerant by capacity exclusion switching” (an idling or compressing conversion technique) is being widely used, in which a portion of a refrigerant gas being compressed in a cylinder is directed outside the cylinder to vary the capacity of the compression chamber.
However, because refrigerant bypasses through the valve, most capacity variable compressors employing the idling or compression conversion technique have the disadvantage of the high resistance of bypass circuit. Therefore, a cooling capability lowering rate in capacity exclusion operation is only 80˜85% of the cooling capability lowering rate in capacity filled operation.
Also, because those compressors cannot speedily switch their operation modes, there is a limit in using them for compressors or air conditioners that require frequent cooling-capability control.